The power spectral density, also referred to as the power spectrum or spectrum, quantifies the frequency distribution of energy or power within a signal. For example, figures 1A–B show representative electroencephalogram spectrograms under general anesthesia maintained with sevoflurane and propofol. In these spectrograms, frequencies are arranged along the y-axis, and time is along the x-axis, and power is indicated by color on a decibel (dB) scale. Figures 1C–D show selected 10-second epochs of raw encephalogram signals from time-points encompassed in figures 1A–B. Figures 1E–L show the 0.1–1 Hz, 1–4Hz, 4–8Hz and 8–14 Hz bandpass filtered electroencephalogram signals from figures 1C–D. We computed spectrograms using the multitaper method, implemented in the Chronux toolbox.21 We computed group-level spectrograms by taking the median across all patients. We also calculated the spectrum for the selected electroencephalogram epochs. The resulting power spectra were then averaged for all epochs, and 95% confidence intervals were computed via multitaper-based jackknife techniques.21 The spectral analysis parameters were: window length T = 2s with 0s overlap, time-bandwidth product TW = 3, number of tapers K = 5, and spectral resolution of 3 Hz. We estimated the peak power, and its frequency, of the frontal alpha oscillation for each individual subject. We then averaged across subjects to obtain the group-level peak power and frequency for these oscillations.